Pulsing vibration at around 55-65 mph

[GMC Guy]

well i figured running it on jack stands would make it easier to tell where the imbalance is coming from. driveshaft, rear-end, axle shafts, or out of round tires.

[GMC Guy]

Originally Posted by Cue-Ball

And yes, a 3" drop is more than enough to screw up your pinion angles.

what does everyone when they lower their truck? i haven't heard any mention of needing to adjust pinion angle when they throw a lowering kit on there. although it makes perfect sense as to why it would mess up the driveshaft angle

[Cue-Ball]

Lowering a vehicle is not a guarantee of having pinion angle issues. Lots of guys lower all sorts of cars/trucks and have no problems. Other guys lower a few inches and everything is out of whack. You might just be one of the unlucky ones.

[GMC Guy]

i'm not really familiar with pinion angle, so could someone let me know how to check/adjust my pinion angle?

[texasbigbear_wink]

You have done the same thing as me spent a lot of time checking everything out and by the time you get dun you could have gotten a new set of tires. The first thing I would have did would of Ban like the other guy said and swap tires with somebody. You can not tell a bad tire just by looking at it. If you have covered everything els like you said you did then I would be about 95% sure it's a bad tire but I could be wrong. I just hope you can figure it out before it cost you and arm and a leg.
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[GMC Guy]

Originally Posted by texasbigbear_wink

You have dun the same thing as me spent a lot of time checking everything out and by the time you get dun you could have gotten a new set of tires. The first thing I would have did would of Ban like the other guy said and swap tires with somebody. You can not tell a bad tire just by looking at it. If you have covered everything els like you said you did then I would be about 95% sure it's a bad tire but I could be wrong. I just hope you can figure it out before it cost you and arm and a leg.
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oh believe me, me too... i don't have an arm and a leg to give lol

[texasbigbear_wink]

I spent $95 on new u joints and balancing the drive shaft, replaced trans mount, inner and outer wheel bearings, fly wheel, had the tires balanced many times thinking they were not dun right, then one day I had a flat that could not be fixt so I had 4 new tires put on and solved a years worth of trying to figure that dam pulsing vibration out.
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[GMC Guy]

Originally Posted by texasbigbear_wink

I spent $95 on new u joints and balancing the drive shaft, replaced trans mount, inner and outer wheel bearings, fly wheel, had the tires balanced many times thinking they were not dun right, then one day I had a flat that could not be fixt so I had 4 new tires put on and solved a years worth of trying to figure that dam pulsing vibration out.
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how old were the tires you replaced? i had these tires put on almost two years ago, and the truck sat about 4 months or so while i was swapping the cabs. but there's no way i have the money to drop on a new set of tires right now. (broke college kid) if i could verify that the tires are the cause of the problem maybe i could just replace the one or two that are bad.

[GMC Guy]

found this article, seems like a shop might charge quite a bit to run these tests on my tires... but it could definitely be the cause of my problems

http://www.aa1car.com/library/tires2.htm

[texasbigbear_wink]

Man they were a little over a year and if it sat 4 months that convinces me even more as to the tires being the problem mine had to sit for a while to when I had an issue with the trans not sure how long that was but I would say 3 to 5 months. I understand not having the money believe me but I just Hait to see you spend money on other things if it is the tires.
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[texasbigbear_wink]

Nice link you found lots of good info.....
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[GMC Guy]

Originally Posted by texasbigbear_wink

Man they were a little over a year and if it sat 4 months that convinces me even more as to the tires being the problem mine had to sit for a while to when I had an issue with the trans not sure how long that was but I would say 3 to 5 months. I understand not having the money believe me but I just Hait to see you spend money on other things if it is the tires.
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believe me if the problem is as simple as the tires i'd be happy. but i just wanna make absolute sure it is the tires before dropping another $500 on a new set

[datguysaL]

yes a simple 3inch drop can cause the vibration.

way to check it is with a angle gauge thingy.. i dunno the correct name to it. but its has a round gauge with a dial in it and 2 straightedges at 90degrees. you take it and hold it up to a flat spot under or above your tranny, preferable within 3 inches from the trans output shaft, then the same on the driveshaft, within 3 inches from the front ujoint, then the same on the rear. about 3inches before the rear ujoint on the shaft and about 3inches from the rear ujoint on the rearend. i think they should all be within 1 degree.

many drop kits are awready designed to change the pinion angle when installed. they also sell shims to correct it.

you said u swappd wheel with your buddy and the problem continued. so chances are your wheels are not the problem.

when exactly did this begin? after you did the drop? after the ujoints? after u got the wheels?

have u check your tranny to make sure its not loose, will the vibration stop if u let go of the gas pedal as soon as u do it or when it drops in speed?

[GMC Guy]

Originally Posted by datguysaL

yes a simple 3inch drop can cause the vibration.

way to check it is with a angle gauge thingy.. i dunno the correct name to it. but its has a round gauge with a dial in it and 2 straightedges at 90degrees. you take it and hold it up to a flat spot under or above your tranny, preferable within 3 inches from the trans output shaft, then the same on the driveshaft, within 3 inches from the front ujoint, then the same on the rear. about 3inches before the rear ujoint on the shaft and about 3inches from the rear ujoint on the rearend. i think they should all be within 1 degree.

many drop kits are awready designed to change the pinion angle when installed. they also sell shims to correct it.

you said u swappd wheel with your buddy and the problem continued. so chances are your wheels are not the problem.

when exactly did this begin? after you did the drop? after the ujoints? after u got the wheels?

have u check your tranny to make sure its not loose, will the vibration stop if u let go of the gas pedal as soon as u do it or when it drops in speed?

funny you mention that, when i let off the gas it seems to mellow out

[GMC Guy]

so i found a "sweet spot" when on the highway. it seems to go down the road pretty smooth at around 75-80 mph (2700-2800 rpm)
it's weird though, all the way up til 48-50 mph it's smooth as well...but i have noticed that for example if going approx. 55-70 and i let off the gas the vibration calms down then picks up as soon as i get back on the gas. i'm picking up a jack today, so i'll check the tranny mount and make sure there is no play. maybe i'll just replace it anyway since i'm sure it's the original. the next thing i'll do is swap around my full size spare with each wheel/tire and see if i can figure out which tire may be causing the problem.

*can anyone give me some tips on replacing the trans mount?

Also, i should mention that i lowered the rear with hangers and shackles. but i have the shackle set on the second hole from the top. hanger is giving me a 2 inch drop and shackle is giving me a 1 inch drop. so could this cause my pinion angle to be off? it appears that if i drop the shackle down to the last notch (4 inch total drop) that the rear-end will level out. adjusting the pinion angle down? i'll try and get a pic to show what i'm talking about

[texasbigbear_wink]

Well first I would get the front on some Jack stands with your truck being low to the ground then remove the nut on the bottom side of the mount first then you can Jack the trans up slowly normally till the stud clears the hole then you will need to remove the two bolts at the top of the mount not sure but I think they are a 15|16 and most of the time you will need a wrench to get to them. I hope that this will help you.
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[GMC Guy]

thanks bigbear, i'm going to get on it tomorrow

[datguysaL]

lets us know if it changes anything wen u do that.

[GMC Guy]

will do man, gosh i really hope it's something simple....

[pkbinplano]

Watching this thread. I have this exact same problem. Feather the gas around 75 mph and get a bad vibration. Going to check mine out this weekend. Had the tranny rebuilt and new carrier bearing and ujoints same time 1200 miles ago. Drive shaft was balanced also. Last time I was under there I noticed the carrier bearing was mounted all the way to one side. Will allign that also.

[pkbinplano]

BINGO !!! Bad fan clutch ! It's loose. The warmer the truck gets the more loose it gets. Vibrates at highway speeds when you feather the pedal like going up a hill.

[texasbigbear_wink]

Pk your lucky on that one.
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[GMC Guy]

well i just finished replacing my driver side lower control arm. see my thread about that. also replaced the tranny mount, wow was that shot...also noticed that my getto exhaust pipes were loose and might have been clanking around. so i got everything back together and tightened up. went for a trip down the highway, and my vibrations have milded down some. still a slight vibration at 55, but smooth over that. also my speedo needle calmed down and read the mph more accurate, probably cuz the tranny is nice and anchored down now lol
so now i'm going to look into the tires more, unless anyone else has some ideas? i'm open for suggestions lol

[triophan]

bump.

I've got the same thing.
tire with 300 miles on them, new 700r4, new cab bushings and core support, trans mount, steering, alignment, pitman arm, balljoints, brakes, lines, etc.
Its getting the shaft balanced and new cups and ujoints come monday.
I need to square this away before I go lowering it (sad too, because the parts are sitting in the shop)

[379redneck]

I had a vibration in the gas pedal at 65. It didnt vibrate the whole truck but it concerned me. If I went faster you could feel it in floor, but that was the worst it got because I didnt want to go faster and realy mess it up. I just happened to check my oil because it leaks, not because of the vibration, and it was very low(below the minimum line). I added oil and the vibration stopped completely. I know it sounds odd and stupid but it was something I never would have thought about and nobody else did either.
Good luck. Hope its nothing major.